An efficient way to produce rotationally asymmetrical surfaces is with a three-axis single point diamond turning lathe. FIG. 1 is an end elevation showing a typical layout for such a lathe and FIG. 2 is a front elevation corresponding to FIG. 1. The lathe which is generally indicated by reference 10 includes a lens supporting assembly 20 and a shuttle 14. The shuttle 14 is axially movable along a “Z axis” indicated by reference Z by an actuator 16. A lens cutting tool 18 (typically a diamond tool) is secured to the shuttle 14.
The lens supporting assembly 20 supports a lens 22 and rotates the lens 22 about a lens axis indicated by θ. The lens supporting assembly 20 is moveable in a direction Y transverse to the lens axis θ. The lens supporting member typically includes a spindle 21 which rotates the lens 20. The spindle 21 is mounted to a transversely moveable linear table 23 which in turn is mounted to a base 25 of the lathe 10.
Lens cutting is effected by a turning operation. The lens 22 is rotated at a high speed about the lens axis θ. The lens cutting tool 18 is initially placed adjacent an edge 24 of the lens 22. The lens 22 is moved in the direction Y as the lens cutting tool 18 is moved in the direction Z. Coordinated movement between the lens 22 and the lens cutting tool 18 determines the shape of the lens 22.
If the lens 22 is rotationally symmetrical, such as spherical or aspherical, the lathe 10 is operated similarly to a two axis turning lathe. The cut typically starts at the edge 24 and the lens cutting tool is moved both in the Y and Z directions (radially inwardly and toward the lens 22). In this instance, the Z position of the lens cutting tool 18 remains constant at any given radial (“Y”) distance from the lens axis θ regardless of rotation about the lens axis θ.
The relative speed between the lens cutting tool 18 and the respective surface of the lens being cut diminishes to zero as the lens cutting tool 18 approaches the lens axis θ. Accordingly, a very high spindle speed in the lens supporting assembly 20 is desirable in order to maintain an acceptable and productive surfacing operation. Typical spindle speeds are on the order of 3,000 to 10,000 RPM.
When the desired lens is non-rotationally symmetrical, as for example in the case of toric or progressive lenses, the lens cutting tool 18 must move reciprocally along the Z axis at a frequency proportional to the rotational frequency. Depending on the particular lens 22 being cut, the lens cutting tool 18 may need to be moved by as much as 20 mm at the edge of the lens. In a simple toric lens this would be a substantially sinusoidal motion with a frequency twice that of the rotational frequency.
A typical actuator 16 would consist of a linear servo motor (such as a voice coil motor) in conjunction with a high speed feedback device which is desirable as being able to produce high speed linear movement at great accuracy. Although such a motor typically has only limited travel, a typical stroke being 30 mm, it may nevertheless be required to achieve velocities as high as 3 to 4 m/s. Such velocities and rapid directional changes can create peak accelerations of 50 to 100 g or even higher. By way of example, if the shuttle 14 and lens cutting tool 18 have a total mass of 2 kg, an actuator acceleration of 100 g will develop reaction forces of 1961 N (approximately 440 lbs).
It will be appreciated that the above velocity and speed figures are somewhat high for currently available linear servo motors. Such technology is rapidly evolving and to some extent the current invention takes into account desired linear servo motor properties. In any case, the present invention produces a useful result with current linear servo motor technology capable of velocities and forces of about half those set out above.
The positioning of the lens cutting tool 18 along its tool path needs to be servo controlled to a very high degree of accuracy, typically within 10 nm or less. Assuming that the actuator 16 is capable of such accuracy, the magnitude of the actuating forces could cause structural defections in the lathe 10 which in themselves exceed the accuracy requirements.
It is an object of the present invention to provide a method and apparatus to cancel vibration caused by actuator forces in a lathe having a reciprocally moveable tool guidance assembly.